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79 620 with v6 hb brake swap proportioning valve?


mbunzel

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Alright guys having some issues with my new brake setup. last week installed belltech 2inch drop spindles only to find the stock brakes didnt work so i ended up doing the hb v6 4x4 brakes which was a much needed upgrade anyway. After installation the brake pedal has a bunch of travel that it didnt use to, but it will stop the truck once the pedal is almost to the ground, and quite well too. Im wondering if I need a different proportioning valve now to compensate for the added pistons up front. Also other things to note the truck has a newer 15/16 master that worked on the stock setup, the booster does still have the valve in the line, and no there is no air in the system.any help would be much appreciated.

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The proportioning valve, I believe, is built into the NLSV (Nissan load sensing valve) It biases less braking to the rear brakes than the front to prevent rear lock up. There is less weight on the rear brakes and when braking weight lifts off the rears and shifts to the front so lock up can be a problem. This bias is built into the system and should not be messed with. The NLSV senses a heavy load in the back and allow more rear braking because of the increased weight/traction on the rear tires.

 

A 15/16" master should work. If all air was removed, then all I can add is adjusting the rear wheel cylinders to move the rear shoes closer to the drums. The farther they have to move the more fluid the master must move.

 

If they are adjusted properly then I have to assume air is still trapped in the system. Did you bleed the NLSM under the passenger seat on the inside of the frame rail???

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I've done a few of these upgrades on 620s and 521s. Not sure if you've done this yet, but check to see that you brake booster push rod is adjusted correctly. If it's too short you'll have a long brake pedal travel before the BMC starts to work.

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The proportioning valve adds rear bias as weight is added to the back of the truck.  

It sure sounds like you have air in the system.  You can test it.  Put the truck on jack stands and have someone spin the wheels while you apply the brakes.  See how far the pedal drops before they slow down or lock up.  You may quickly find your problem.  

 

If you went from drums to discs, then you may need to add a 10 psi residual pressure valve for the rear brake line, not for the front discs.  It will hold fluid in the rear lines for the wheel cylinders yet allow pressure to be dropped 100% from the calipers.  Wilwood makes them.  A 79/80 master will be set up this way, and probably others.  

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If you went from drums to discs, then you may need to add a 10 psi residual pressure valve for the rear brake line, not for the front discs.  It will hold fluid in the rear lines for the wheel cylinders yet allow pressure to be dropped 100% from the calipers.  Wilwood makes them.  A 79/80 master will be set up this way, and probably others.  

 

I think it's the other way round. Drums need about 10 PSI residual valves, rear disc only 3 PSI or so.

 

I have a 15/16 zx master in my rear drum 710. I just swapped the 710 rear drum residual valve into the zx master.

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